(1) Field of the Invention
The present invention relates to a digital signal transmission method of minimizing quality deterioration in high-speed radio data transmission where severe degradation is caused by a multipath propagation in urban areas or around/inside of buildings frequently.
(2) Description of the Related Art
Recently, the needs of the high-speed radio data transmission, such as a radio LAN (Local Area Network), are increasing rapidly.
The digital radio transmission in urban areas or inside of buildings, however often has the problem of degradation in transmission quality due to multipath fading caused by reflections and diffractions by buildings or walls. Particularly when the propagation delay time difference between waves increases to such an extent that it is no longer negligible with respect to the length of a time slot (symbol period), the bit error rate (BER) characteristics are severely affected by signal distortion.
As a conventional method of combatting the BER degradation, there is a scheme in which a redundant amplitude/phase transition is imposed on a basic modulation.
The conventional digital signal transmission method will be described below as referring to the drawings.
FIG. 1 shows the phase transition of a transmission signal according to the conventional digital signal transmission method. The information to be transmitted is present in a phase difference .theta. between waveforms in neighboring time slots; and a convex phase transition .phi.(t) is redundantly imposed on each time slot. (Hitoshi Takai, "BER Performance of Anti-Multipath Modulation Scheme PSK-VP and its Optimum Phase-Waveform, IEEE Trans. VT, vol 42, No. 4, pp. 625-640, November 1993). That is, a convex phase transition .phi.(t) is imposed on a DPSK (Differently encoded Phase Shift Keying). (For reference, FIG. 2 shows a differential encoding PSK without the imposing). The transmission signal is detected by a differential detector using a delay line with a delay of one time slot (symbol).
The improvement in BER characteristics in the presence of a two-wave multipath having a propagation time difference .tau. will be described as referring to FIGS. 3 and 4 (a wave arriving earlier is called as a "direct wave" and a wave arriving succeeding to the direct wave is called as a "delayed wave").
FIG. 3 shows the detected output under two-wave multipath in DPSK.
The unfiltered detected output (solid line) in FIG. 3 (c) is obtained via the vectorial sum of the direct and delayed waves having the phase waveforms of FIG. 3 (a) and (b). The detailed detecting method will be described below. The waveform of a final detected output (dotted line) is obtained by filtering the unfiltered detected output. The final filtered detected output is sampled at a timing corresponding to symbol period; its polarity is judged; and it is decoded into binary data (refer to FIGS. 14-16 for further explanation of the decoding process). In a time area where the direct waveform symbol and the corresponding delayed waveform symbol are overlapped with each other (hereunder referred to as an effective area), polarity of the detected output before filtering is always correct.
However, as shown in FIG. 3, when the direct wave and the delayed wave are opposite to each other in phase, an effective output becomes significantly small. Accordingly, amplitude of the unfiltered detected output (solid line) decreases remarkably. After filtering, the adjacent ineffective output having a relatively large amplitude is mixed into the effective output, and the BER characteristics are degraded severely.
Like FIG. 3, FIG. 4 shows the detected output under the two-wave multipath in the case of the conventional digital signal transmission method explained in FIG. 1. Different from FIG. 3, the unfiltered effective output (solid line in FIG. 4 (c)) is not constant because of the imposed redundant phase.. It is apparent from a vector diagram in FIG. 4 (d) that the direct and delayed waves do not cancel each other except for a part of the effective area. Therefore, some of the detected output from the effective area remains after filtering (dotted line). As a result, the BER characteristics will not be degraded.
According to the conventional digital transmission method, however, a phase discontinuity occurs between time slots (symbols). Therefore, if the signal is band-limited so as to keep .a transmission spectrum narrow, the envelop of signals varies, and the BER characteristics tend to be degraded because when passing the transmission signal through a non-linear amplifier, amplitude/phase conversion characteristics of the amplifier causes a phase-distortion.
Besides, if a non-linear amplifier is used, the band-limitation for spectrum compactness is ineffective because the transmission spectrum will be expanded again when the transmission signal passes through the non-linear amplifier. The transmission spectrum could be kept compact by using a highly linear amplifier; however, a highly linear amplifier is expensive, and it is poor in electric-power efficiency.